This invention is directed to the fabrication of unitary structures, such as missile radomes, from fiber reinforced plastic material. More particularly, this invention relates to radomes which are resistant to ablation and rain erosion, particularly at high operating speeds such as Mach 4 or higher.
Ceramic radomes are typically used for missiles intended to operate at speeds of Mach 4 or higher. These ceramic radomes have been found to be marginal in performance due to fragility, susceptibility to thermal shock, high thermal conductivity, high rates of rain impact damage. A definite need exists for a workable alternative to ceramic radomes.
Radomes made from polymeric composite materials have been suggested as a possible alternative to ceramic radomes. Polytetrafluoroethylene, hereinafter PTFE, is one such polymeric material which might be suitable for radome applications. However, "neat" or simple filled PTFE does not possess the requisite characteristics, uniformity of erosion and ablation for example, for use in the demanding environment of a missile radome. Tests have shown that fiber filled PTFE; i.e., a PTFE composite; would have those characteristics dictated by radome and similar usage.
Prior to the present invention it has been a practical impossibility to fabricate a radome from a PTFE-fiber composite. The production of a solid block of a PTFE composite of sufficient size to permit machining a radome therefrom is not feasible due to the virtual impossibility of heating such a large block through the crystalline melt point and subsequently cooling through the recrystallization point with enough uniformity of temperature to avoid fissures and damage from thermal stress. Further, even if the temperature gradient and thermal stress problems could be avoided, an extremely long heating and cooling cycle (perhaps on the order of several weeks) would be required, and that long cycle time would result in thermal degradation. Other approaches, such as flowing a sheet of PTFE composite material to form a radome shape or laminating a series of rings or discs cut from such sheet material all involve substantial technical or cost problems which have previously precluded the use of such material and techniques.